Microelectronic assemblies with composite conductive elements

ABSTRACT

A microelectronic assembly includes composite conductive elements, each incorporating a core and a coating of a low-melting conductive material. The composite conductive elements interconnect microelectronic elements. At the normal operating temperature of the assembly, the low-melting conductive material melts, allowing the cores and microelectronic elements to move relative to one another and relieve thermally-induced stress.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This is a divisional application of U.S. patent application Ser.No. 09/243,860 filed Feb. 3, 1999, which in turn claims benefit of U.S.Provisional Patent Application No. 60/073,520, filed Feb. 3, 1998.

BACKGROUND OF THE INVENTION

[0002] The present invention relates to components and methods formounting and connecting microelectronic elements such as semiconductorchips.

[0003] Complex microelectronic devices such as semiconductor chipsrequire numerous connections with other electronic components. Typicallysuch semiconductor chips are mounted on external substrates such asprinted circuit boards by electrically interconnecting contacts on thesemiconductor chip with contact pads on the substrate. The substrate mayinclude internal circuitry which is connected to the contact padsthereof and may be adapted to accommodate other components such asadditional semiconductor chips.

[0004] Connections between microelectronic elements and substrates mustmeet several demanding and often conflicting requirements. Theconnections must provide reliable low impedance electricalinterconnections. They must also withstand stresses occurring duringmanufacturing processes, such as the thermal effects caused bysoldering. Other thermal effects occur during operation of the device.As the system operates, it generates heat which causes the chip and thesubstrate to expand. When operation ceases, the chip and the substratecool down which causes the components to shrink or contract. As aresult, the chip and the substrate expand and contract at differentrates so that portions of the chip and substrate move relative to oneanother. The chip and the substrate can also warp as they are heated andcooled, thereby causing further movement of the chip relative to thesubstrate. The repeated expansion and contraction of the elements duringoperation results in severe strain on electrical elements connecting thechip and the substrate.

[0005] In response to these problems, various interconnection systemshave evolved. These systems essentially seek to withstand repeatedthermal cycling without breaking the electrical connections. Theinterconnection system should also provide a compact assembly and shouldbe suitable for use with components having closely spaced contacts.Moreover the interconnection system should be economical. Varioussolutions have been proposed to meet these needs. Some embodiments ofcommonly assigned U.S. Pat. Nos. 5,148,265 and 5,148,266 teach thatflexible leads may be provided between the contacts on a semiconductorchip or other microelectronic element and contact pads of a substrate.Preferably a compliant layer, such as an elastomer or gel, may beprovided between the semiconductor chip and the substrate, whereby theflexible leads connecting the semiconductor chip and substrate extendthrough the compliant layer. In certain preferred embodiments, thesemiconductor chip is mechanically decoupled from the substrate so thatthe chip and the substrate can expand and move independently of oneanother without placing excessive stress on the electrical connections(i.e. the flexible leads) between the chip contacts and the contact padsof the substrate. The chip and the flexible leads extending to thesubstrate typically occupy an area of the substrate about the same sizeas the chip itself.

[0006] Commonly assigned U.S. patent application Ser. No. 08/641,698,the disclosure of which is hereby incorporated by reference herein,discloses connection components for microelectronic assemblies. Inaccordance with certain preferred embodiments of the '698 application,the microelectronic assemblies preferably include first and secondmicroelectronic elements having contacts thereon and a compliantdielectric material having cavities therein. Masses of a conductivematerial are disposed in the cavities so that the masses of theconductive material are electrically interconnected between contacts onthe first microelectronic element and contacts on the secondmicroelectronic element. Thus, each conductive mass forms part or all ofa conductor extending between contacts on the two microelectronicelements. The conductive material may be a liquid or may be a fusiblematerial adapted to liquefy at a relatively low temperature, typicallybelow about 125° C. Preferably the conductive material in each mass iscontiguous with the compliant material and is contained by the compliantmaterial so that the conductive material remains in place when in aliquid state. The compliant material keeps the liquid masses associatedwith different sets of contacts separate from one another andelectrically insulates the masses from one another.

[0007] Commonly assigned U.S. patent application Ser. No. 08/962,693entitled “Microelectronic Connections with Liquid Conductive Elements,”filed Nov. 3, 1997 (as a CIP of the above-mentioned '698 application),the disclosure of which is hereby incorporated by reference herein,discloses microelectronic assemblies having conductive elements whichtransfer heat between microelectronic elements. In certain embodiments,first and second microelectronic elements are juxtaposed with oneanother so that the confronting spaced apart surfaces of the first andsecond microelectronic elements define a space therebetween. One or moremasses of a conductive material having a melting temperature below about150° C. (hereinafter referred to as “fusible conductive material”) areprovided in the space. The fusible conductive material is preferablythermally conductive, electrically conductive or both. A compliant layeris provided in the space between the microelectronic elements. Thefusible conductive masses are preferably contained by the compliantlayer and may extend between contacts on the confronting surfaces of thefirst and second microelectronic elements so that the masseselectrically interconnect the first and second microelectronic elements.The fusible conductive masses also preferably provide a thermalconduction path between the first and second microelectronic elements.

[0008] Commonly assigned U.S. patent application Ser. No. 08/862,151filed May 22, 1997, the disclosure of which is hereby incorporated byreference herein, discloses connectors for microelectronic elements. Incertain preferred embodiments of the '151 Application, a microelectronicelement is engaged with a connecting assembly. The connecting assemblypreferably includes a flexible dielectric interposer, a substrate andnon-collapsible structural elements which support the flexibledielectric interposer above the substrate, leaving a standoff spacebetween the dielectric interposer and the substrate. In one preferredembodiment a microelectronic element having a plurality of contactsprotruding from the bottom surface thereof is engaged with the flexibledielectric interposer. The contact bearing surface of themicroelectronic element is juxtaposed with the top surface of theflexible dielectric interposer whereby the contacts of themicroelectronic element are generally in registration with an array ofcontacts provided on the flexible dielectric sheet. The microelectronicelement is then urged downwards so that contacts engage the contact padsof the flexible dielectric sheet. Downward motion of the microelectronicelement relative to the flexible dielectric sheet resiliently deformsthe flexible sheet with each microelectronic element contact resilientlydeflecting a surrounding portion of the flexible sheet downward into astandoff space between the flexible sheet and the substrate. As thesurrounding portion of the sheet-like element is deflected into thestandoff space, the flexible sheet is stretched. The structural elementsoverlying the substrate force the flexible sheet-like element upward asthe microelectronic element contacts force the sheet-like elementdownward.

SUMMARY OF THE INVENTION

[0009] One aspect of the present invention provides improvedmicroelectronic assemblies. A microelectronic assembly according to thisaspect of the invention preferably includes first and secondmicroelectronic elements and a plurality of composite conductiveelements. Each composite conductive element includes a core and a layerof a conductive material surrounding the core. The conductive materialpreferably has a melting temperature less than about 150° C., whereasthe cores have a melting temperature higher than the melting temperatureof the conductive material. The conductive elements are disposed betweenthe microelectronic elements and connect the microelectronic elements toone another. Thus, the composite conductive elements typically conductelectrical signals, heat or both between the microelectronic elements.

[0010] Typically, the microelectronic elements have a normal operatingtemperature range and the melting temperature of said conductivematerial is within or below this normal operating temperature range, oronly slightly above such range. Therefore, during operation of theassembly, the conductive material wholly or partially liquefies. Thecores desirably have melting temperatures well above the operatingtemperature range of the microelectronic elements, so that the materialconstituting the cores remains solid during normal operation. The liquidconductive material allows movement of the cores relative to themicroelectronic elements, and allows movement of the microelectronicelements relative to one another. This provides compensation fordifferential thermal effects and other effects which tend to causerelative movement of the elements during service. For example, the firstmicroelectronic elements may be a semiconductor chip and the secondmicroelectronic element may be a circuit panel or a packaging elementwhich is connected to a circuit panel in service. The molten layers ofthe composite conductive elements will allow movement of the circuitpanel relative to the chip. Moreover, the connection remainsfatigue-free.

[0011] Typically, the layer of conductive material in each conductiveelement is about 50 μm or less thick, and the cores are spherical. Thethin conductive layer reduces the amount of relatively expensivelow-melting alloy required in the assembly. The spherical coresfacilitate movement of the microelectronic elements relative to oneanother when the conductive material is in a molten condition. Althoughthe present invention is not limited by any theory of operation, it isbelieved that the spherical cores roll on one or both microelectronicelements, and thus act much like miniature bearing balls. The cores canbe formed from essentially any material. In a particularly desirablearrangement, the cores may be readily deformable to facilitate verticalor Z-direction movement of the microelectronic elements relative to oneanother in service. The cores may be solid throughout or else may behollow. Preferably, the assembly also includes a filler disposed betweenthe microelectronic elements and surrounding the conductive elements.The filler can help to constrain the liquid conductive material when theconductive material is in the molten state. The filler desirably is acompliant material such as a gel, elastomer, foam or mesh so that thefiller can deform to accommodate the relative movement of themicroelectronic elements.

[0012] In a particularly preferred arrangement, the secondmicroelectronic element includes a flexible film having a first surfacefacing in a first vertical direction toward the first microelectronicelement and having a second surface facing in a second verticaldirection away from said first microelectronic element. The film hascontacts on its first surface, which are connected to contacts on thefirst microelectronic element by the conductive elements. The film alsohas contacts on its second surface electrically connected to thefirst-surface contacts and conductive elements. The second surfacecontacts may be offset from the first-surface contacts and conductiveelements in one or more lateral directions transverse to the verticaldirections. In this arrangement, the film can bend or bow to accommodatevertical movement of the second-surface contacts relative to the firstmicroelectronic element. Stated another way, the assembly provides goodZ-direction compliance, which facilitates mounting of the assembly andalso facilitates testing of the assembly prior to mounting. The firstmicroelectronic element may be a semiconductor chip having a frontsurface facing toward the film or second microelectronic element.

[0013] Another aspect of the invention provides methods of operating amicroelectronic assembly having conductive elements connecting first andsecond microelectronic elements. A method according to this aspect ofthe invention desirably includes the step of bringing said assembly toan operating temperature above the melting temperature of a conductivematerial incorporated in the conductive elements but below the meltingtemperature of cores in the conductive elements. In this condition, thecores are free to move relative to the microelectronic elements and themicroelectronic elements are free to move relative to one another. Thecores and conductive material conduct electrical signals, heat or bothbetween the microelectronic elements while the microelectronic elementsare at operating temperature. One or both of the microelectronicelements may include a flexible sheet and the flexible sheet may deformduring operation. Movement of the cores allowed by the molten conductivematerial accommodates deformation of the flexible sheet. A filler suchas a compliant material surrounding the conductive elements mayconstrain the conductive material while the conductive material is in amolten condition. Desirably, the filler maintains the conductiveelements in compression when the assembly is in its normal range ofoperating temperatures. For example, the coefficient of thermalexpansion (“CTE”) of the filler may be equal to or greater than the CTEof the cores in the conductive elements, and the filler may be appliedand cured at a temperature at or near the top of the normal operatingtemperature range.

[0014] Yet another aspect of the invention provides conductive elements.A conductive element according to this aspect of the invention desirablyincludes a spherical core and a layer of a conductive material on thecore, the conductive material having a melting temperature less thanabout 150° C., and typically less than 85° C. The core has a meltingtemperature above the melting temperature of the conductive material.Conductive elements according to this aspect of the invention can beused in assemblies and methods as discussed above.

[0015] These and other objects, features and advantages of the presentinvention will be more readily apparent from the detailed description ofthe preferred embodiment set forth below and taken in connection withthe accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a diagrammatic sectional view depicting an assemblyaccording to one embodiment of the invention during one stage of itsmanufacture.

[0017]FIG. 2 is a view similar to FIG. 1, showing the same assembly atafter completion of manufacture.

[0018]FIG. 3 is a view similar to FIG. 1, showing the same assembly inuse.

[0019]FIG. 4 is a diagrammatic sectional view depicting an assemblyaccording to another embodiment of the invention.

[0020]FIG. 5 is a view similar to FIG. 4 depicting an assembly accordingto yet another embodiment of the invention.

[0021]FIG. 6 is a diagrammatic sectional view depicting an assemblyaccording to a further embodiment of the invention.

[0022]FIG. 7 is a view similar to FIG. 6 but depicting an assembly inaccordance with yet another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] One embodiment of the present invention provides amicroelectronic assembly 20 (FIG. 1) having composite conductiveelements 22. The microelectronic assembly 20 preferably includes a firstmicroelectronic element 24, such as a semiconductor chip, having aplurality of contacts 26 on a front face 28 thereof. The assembly 20also includes a second microelectronic element 30, such as a flexibledielectric film, having first contacts 32 on a first surface 34 thereof.The dielectric film 30 may also have second contacts 36 on a secondsurface 38 thereof which are in registration with the first contacts 32.A plurality of the composite conductive elements 22 are preferablyprovided over the contacts 32 on the first surface 34 of the dielectricfilm 30. Each composite conductive element 22 desirably includes a core40 which is preferably in the shape of a sphere. The core 40 may includea conductive material, such as metal, or a dielectric (non-conductive)material, such as an elastomer. The core 40 may be entirely solidthroughout or may comprise a hollow sphere. Moreover, the core 40 may besubstantially rigid or compliant. Although the cores can be ofessentially any size, each core preferably is between about 0.15 and 1.5mm and more desirably between about 0.25 mm and about 0.5 mm indiameter. All of the cores used in a particular assembly should be ofsubstantially the same diameter.

[0024] Each composite conductive element 22 includes a layer of aconductive material 42 which surrounds the core 40. The layer ofconductive material 42 is preferably about 25-50 microns or less. Thelayer of the conductive material 42 preferably conducts electricity andheat; however, in other embodiments the conductive material 42 may onlyconduct electricity. In still further embodiments the conductivematerial 42 may only conduct heat. In other words, although thecomposite conductive elements 22 are preferably used to simultaneouslyconduct both electricity and heat, the composite conductive elements 22may be used in certain embodiments to conduct only heat. For example,the composite conductive elements 22 may be disposed between the rearsurface of a semiconductor chip and a heat sink for conducting heattherebetween.

[0025] The melting temperature of the conductive material (hereafterreferred to as “fusible conductive material”) desirably is within orbelow the normal operating temperature of the microelectronic elementsin the assembly 20, or only slightly above the normal operatingtemperature range. The normal, expected range of operating temperaturesof the microelectronic elements will depend upon the configuration andcomposition of the assembly 20, and upon the environment encounteredduring operation. Typical silicon-based microelectronic elements aredesigned to operate at about 40° C. to about 85° C. Where the fusibleconductive material 42 melts or freezes over a range of temperatures,the term “melting temperature” as used in this disclosure should beunderstood as referring to the solidus temperature, i.e., thetemperature at which the fusible conductive material 42 begins to melt(when heated slowly) or completes freezing (when cooled slowly).Preferably, the melting temperature of the fusible conductive material42 is above normal room temperature (20° C.) so that the fusibleconductive material 42 can be handled conveniently in solid form duringassembly steps. Thus, the fusible conductive material 42 may have amelting temperature of less than about 150° C., preferably less thanabout 125° C., more preferably less than about 100° C., even morepreferably less than about 85° C. and most preferably less than about65° C. For fusible conductive material having melting temperatures below65° C., temperatures between about 25° C. and 65° C. are particularlypreferred, and melting temperatures between about 35° C. and about 55°C. are most preferred. However, lower melting temperatures can beemployed if the production process is altered to accommodate the lowermelting temperature. For example, where a composite conductive element22 includes a fusible conductive material 42 which melts at atemperature below room temperature is employed, the fusible conductivematerial 42 and the adjacent parts can be kept at sub-ambienttemperatures during those process steps so that the fusible conductivematerial 42 remains in a solid state. Conversely, where the operatingtemperature of the microelectronic elements is higher than the typicalranges mentioned above, fusible conductive materials which melt athigher temperatures can be employed.

[0026] Among the suitable low-melting point materials which may be usedas the layer of fusible conductive material 42 are the following soldercompositions: COMPOSITION 1 COMPOSITION 2 ELEMENT WEIGHT % WEIGHT % Sn  18.5 10.5 Bi 45 40   Pb 24 21.5 In 10 20   Cd   9.5 8  MeltingTemperature 55° C. 50° C.

[0027] Solders having compositions intermediate between the twolow-melting point solders illustrated in Table 1 can be used. Othersuitable low-melting solders include the solder sold under the trademarkIndalloy by the Indium Corporation of America, located in Clinton, N.Y.For example, Indalloy Number 8 has a melting point of about 93° C.,whereas Indalloy Number 117 has a melting point of about 47° C. Stillother low-melting solders include other combinations of metals selectedfrom the group consisting of cadmium, bismuth, tin, lead and indium invarious proportions, with or without other metals. Additional fusibleconductive materials include gallium, mercury and mercury containingalloys.

[0028] After the composite conductive elements 22 are connected to thecontacts 32 on the first surface 34 of the flexible dielectric sheet 30,the semiconductor chip 24 is assembled to the dielectric sheet so thatthe first surface 34 of the flexible sheet 30 faces the front face 28 ofthe semiconductor chip 24, the confronting faces 28 and 34 defining aspace 44 therebetween. The contacts 26 on the semiconductor chip 24 arepreferably aligned with the contacts 32 on the first surface of thedielectric sheet 30 and with the composite conductive elements 22disposed on the dielectric sheet contacts 32. The alignment between thechip contacts 26 and the composite conductive elements 22 need not beperfect. The alignment need only be close enough so that each chipcontact 26 touches the correct composite conductive element 22 duringthe melting step discussed below so that each chip contact 26 does nottouch any other composite conductive element.

[0029] While the chip contacts 26 are held in engagement with thecomposite conductive elements 22, the layer of the fusible conductivematerial 42 surrounding the core 40 of each composite conductiveelements 22 is brought to a temperature above its melting temperature sothat the fusible conductive material layer 42 at least partiallyliquefies and flows into intimate engagement with the exposed surfacesof the chip contacts 26. Alternatively, the dielectric sheet 30 and thecomposite conductive elements 22 may be at a temperature above themelting temperature of the layer of the fusible conductive material 42prior to engagement with the semiconductor chip 24. The chip contacts 26may also be preheated to a temperature above the melting temperature ofthe fusible conductive material 42 before engagement with the compositeconductive elements 22.

[0030] The fusible conductive material 42 preferably wets to thesurfaces of the chip contacts 26. While the fusible conductive material42 is in at least a partially molten condition, a plate (not shown)holds the flexible dielectric sheet 30 in a substantially planarcondition so that the second contacts 36 on the second surface 38 of theflexible dielectric sheet 30 are substantially co-planar with oneanother. While the second contacts 36 are aligned in this manner, thecomposite conductive elements 22 are cooled to below the meltingtemperature of the fusible conductive material 42, such as by coolingthe entire microelectronic assembly including the supporting plates. Inother embodiments, the flexible dielectric sheet 30 may be stretchedand/or held taut using a substantially rigid ring (not shown). Forexample, the dielectric sheet 30 may be stretched in the ring in themanner disclosed in U.S. Pat. No. 5,518, 964, the disclosure of which ishereby incorporated by reference herein and a copy of which is annexedhereto.

[0031] Referring to FIG. 2, after the composite conductive elements 22assembled between the semiconductor chip 24 and the flexible dielectricfilm 30 and have been completely frozen, a flowable liquid material 46is introduced into the space 44 between the chip 24 and the flexiblesheet 30 so that the flowable material 46 fills the space 44 andintimately surrounds the composite conductive elements 22 and theadjacent surfaces of contacts 26 and 32. The flowable material alsointimately engages the front face 28 of the semiconductor chip 24 andthe first surface 34 of the flexible dielectric film 30. Duringintroduction of the flowable material 46, the dielectric film 30 ispreferably maintained in a planar condition by either the supportingplate or the frame described above. After the space 44 has beencompletely filled by the flowable material 46, the flowable material 46is cured to form a compliant layer occupying the space 44 and intimatelysurrounding the composite conductive elements 22 and the contacts 26 and32. The compliant layer 46 may be a solid or a gel and preferablycomprises a silicone elastomer or a flexibilized epoxy. The compliantlayer 46 is preferably CTE matched with the composite conductiveelements 22 to minimize the effects of thermal cycling. The flowablematerial 46 is preferably introduced into the space 44 and cured at atemperature which is below the melting temperature of the fusibleconductive material 42. In further embodiments, the flowable material 46may be introduced around the composite conductive elements 22 when thefusible conductive material 42 surrounding the cores 40 is in a liquidstate. The fusible conductive material 42 may then be frozen before theflowable material cures or after the flowable material cures.

[0032] In further embodiments, the CTE of the cured flowable material orfiller, and the curing temperature, are selected so that when theassembly is in the normal operating temperature range, the filler willmaintain the conductive elements in compression. Thus, where the curedflowable material or filler 46 has a higher coefficient of thermalexpansion than the composite conductive elements, the filler preferablyis cured at a temperature which is at or near the upper end of theoperating temperature range of the assembly. As a result, duringoperation of the assembly the composite conductive elements will beunder compression, thereby ensuring that the composite conductiveelements maintain a reliable electrical interconnection between the chipand the dielectric sheet. If the cured filler has a lower CTE than thecomposite conductive elements, curing preferably occurs at a temperaturebelow the normal operating temperature range, or at the low end thereof.

[0033] Referring to FIG. 3, the finished microelectronic assembly 20 maythen be connected to an external circuit element 48, such as a printedcircuit board, by juxtaposing the second contacts 36 on the secondsurface 38 of the flexible dielectric film 30 with contact pads 50 onthe printed circuit board 48. The flexible dielectric film 30 and theprinted circuit board 48 are then electrically interconnected with aconductive material 52 such as solder. During operation of the assembly20, the layer of the fusible conductive material 42 of each compositeconductive element 22 will preferably liquefy at normal operatingtemperatures. When the fusible conductive material 42 has obtained aliquid or molten state, the cores 40 will be free to move in lateraldirections, thereby providing the assembly 20 with excellent X-axis andY-axis compliance, i.e., compliance allowing movement of the dielectricfilm 38 and contacts 32 thereon relative to chip 24 and contacts 26.This minimizes stress on the solder bonds 52 between the contacts padsof the substrate and the remainder of the assembly caused bydifferential thermal expansion and contraction. Depending upon thenature of cores 40, the assembly may also have substantial Z-directioncompliance, i.e., compliance in directions perpendicular to the chipsurface. Thus, where cores 40 are compressible, as where the cores areformed from elastomeric material, or are hollow, thin-walled flexiblestructures, the assembly will have appreciable Z-direction compliance.Similarly, the compliance of the assembly can compensate for thermaleffects during manufacture. The conductive material layers will liquefyduring the soldering operation, and will allow movement of thedielectric film and contacts during heating and cooling associated withthe soldering operation.

[0034] This embodiment of the present invention provides microelectronicassemblies having excellent compliance so as to enhance the reliabilityof the electrical connections between microelectronic elements. Inaddition, the microelectronic assemblies may be manufactured in aneconomical and efficient manner. The composite conductive elements ofthe present invention are relatively inexpensive because the coresthereof typically include a relatively inexpensive material such ascopper, elastomers (e.g. silicone elastomers) or plastics. The cores maybe relatively simple and inexpensive because there is no need for thecore to melt during operation; only the thin outer layers of thecomposite conductive elements must melt in order to provide adequateelectrical and thermal interconnections. As a result, the compositeconductive elements of the present invention may provide reliableconductive means while reducing the overall cost of the assembly. Inaddition, each composite conductive element has a core which facilitateshandling and placement of the conductive balls. As a result of the core,each composite conductive element can be handled and placed even whenthe fusible material is liquid or molten because surface tension willhold the layer of fusible material around the core. In contrast, whenthe conductive balls completely comprise a fusible material, theconductive balls cannot be handled while in a molten or liquid state.Still further, because the amount of low-melting fusible material ineach conductive element is minimal, any adverse effects of suchmaterial, such as alpha particle emission from heavy elements containedtherein, are also minimized.

[0035] Referring to FIG. 4, a microelectronic assembly 120 in accordancewith another preferred embodiment of the present invention is fabricatedusing materials and methods substantially similar to those describedabove. However, in this particular embodiment the second contacts 136 onthe second surface 138 of the flexible dielectric sheet 130 are offsetin a lateral direction from the first contacts 132 on the first surface134 of the flexible dielectric sheet 130. For example, the where thefirst contacts are disposed in a grid pattern on one side of the sheet,the second contacts can be disposed in a similar grid pattern on theopposite side of the sheet. The second grid pattern may have the samecenter-to-center distances as the first grid pattern, but may be offsetfrom the first gird pattern. Alternatively, the second contacts may be“fanned” from the first contacts. Thus, where the contacts on the chipare disposed in a small region such as a line or small pattern, thefirst contacts 132 are disposed in the same region. The second contactsmay be disposed in a larger array, covering a larger region of the sheetsurface. Such an arrangement is commonly referred to as a “fan-out” fromthe chip contacts. In still other embodiments, the chip contacts andhence the first contacts on the sheet may be disposed adjacent theperimeter of the chip, whereas the second contacts may be disposed in anarray within the perimeter. Such an arrangement is commonly referred toas a “fan-in”. Combinations of these approaches can be used. The secondcontacts 136 on the second surface are electrically connected to thefirst contacts. In FIG. 4, these connections are exemplified as traces137 extending from first contacts 132 along a surface of sheet 130 andvias extending from traces 137 to second contacts 136. This arrangementis merely illustrative, inasmuch as the flexible sheet may include oneor more layers of traces on its surfaces or within the sheet, and mayalso include other elements such as flexible conductive planes,typically used as ground and power planes. As such, this particularembodiment of the present invention provides, in addition to lateralcompliance along the X- and Y- axes, outstanding vertical Z-axiscompliance even if cores 140 are rigid. As mentioned above, duringoperation of the assembly 120, the semiconductor chip 124 and theexternal circuit element 148 heat up and expand. This may cause thecontact pads 150 on the external circuit element 148 to move towards thesemiconductor chip 124 and the contacts 126 on the front face 128 of thesemiconductor chip 124 to move towards the external circuit element 148.For example, such movement may occur if the parts warp upon expansion.The flexible sheet 130 underlying the chip contacts 126 can movedownward and the flexible sheet 130 overlying the contact pads 150 onthe circuit element 148 can move upward. As the respective contactsmove, the flexible sheet 130 bends. This bending, and deformation ofcompliant layer 146 allows the parts to move relative to one another inthe Z-direction which further minimizes the effects of thermal cycling.In addition, when the chip 124 heats up, the layer of the fusibleconductive material 142 of each conductive element 122 melts which freesthe core 140 thereof to move in lateral (X and Y-axis) directions. Thisalso facilitates bending of the sheet and Z-axis compliance. Theresilience of the flexible sheet 130 and the compliant layer 146, inconjunction with the fusible conductive material 142, maintainselectrical contact between the semiconductor chip 124 and the externalcircuit element 148. The Z-axis compliance afforded by this and otherembodiments of the invention can also be used to facilitate testing ofthe assembly. Thus, where the second-surface contacts 136 are engagedwith a rigid test fixture (not shown), the Z-axis compliance will allowall of the contacts to engage the fixture even if the contacts or thefixture are not perfectly coplanar.

[0036] In a variant of the approach shown in FIG. 4, the compositeconductive elements are replaced by conventional solder balls or otherconnections which remain rigid during operation. Here again, the firstcontacts and the connections between the first contacts and the chip arelaterally offset from the second contacts and the connections betweenthe second contacts and the external circuit element 148. Also, thesheet 130 is flexible and held away from the external circuit elementand chip by the connections. Under these conditions, flexure of sheet130 can still allow some movement of the chip relative to the externalcircuit element, particularly in the Z or vertical direction. In such anembodiment, the space between the sheet and chip may be provided with afiller, which desirably is compliant. A similar filler may be providedbetween the sheet and the external circuit element.

[0037] Referring to FIG. 5 in still further preferred embodiments of thepresent invention, a microelectronic assembly 220 may be fabricatedusing techniques substantially similar to those described above.However, in these particular preferred embodiments a first set ofcomposite conductive elements 222A are provided between the front face228 of the semiconductor chip 224 and the first surface 234 of theflexible dielectric element 230. A second set of composite conductiveelements 222B are provided between the second surface 238 of theflexible dielectric sheet 230 and the contact pads 250 of the externalcircuit element 248. The composite conductive elements 222A and 222Bpreferable provide electrical and thermal connections between thesemiconductor chip 224 and the circuit board 248. The assembly 220 alsopreferably includes a compliant interface which minimizes the stresseson the electrical connections during thermal cycling so as to maintainthe electrical connections between the elements. As mentioned above, thecomposite conductive elements 222A and 222B electrically interconnectthe semiconductor chip 224 and the external circuit element 248. Whenthe assembly 220 has reached a predetermined operating temperature, thefusible conductive material layer 242 surrounding the core 240transforms into a liquid or molten state, thereby providing the assembly220 with excellent vertical and lateral compliance. The compositeconductive elements also preferably conduct heat so that heat may bedissipated from the assembly.

[0038] In certain embodiments, the second microelectronic element mayinclude a substantially rigid substrate. Although a rigid substratewould provide for minimal Z-compliance, the assembly would still providefor lateral compliance.

[0039] Referring to FIG. 6, in certain preferred embodiments a polymersuch as a polyparaxylene coating 354 (also referred to by the trade nameParalyene) may be provided over the composite conductive elements 322.The polyparaxylene coating 354 is preferably a conformal coating whichfully encompasses the composite conductive elements 322 and the adjacentregions of the contacts 326 on the front face 328 of the semiconductorchip 324 and the first surface 334 of the flexible sheet 332. Thepolyparaxylene coating 354 extends between the respective confrontingsurfaces 334 and 328 of the flexible dielectric sheet 332 and thesemiconductor chip 324 and may cover portions of the confrontingsurfaces 334 and 328 as well. The coating 354 helps to maintain thecomposite conductive elements 322 in place when the fusible conductivematerial 342 is in a molten condition and also helps to preserve theelectrical isolation of the composite conductive elements 322 from oneanother. The polyparaxylene coating may also prevent cross-contaminationof the fusible conductive material 342 and the compliant layer 346. Inother words, the coating 354 may prevent the fusible conductive material342 and the material of a compliant layer 346 from diffusing into oneanother.

[0040] An assembly according to yet another embodiment of the invention(FIG. 7) includes a first element in the form of a semiconductor chip424 and a second element 434 in the form of a rigid interposer havingfirst contacts 432 facing toward the chip and second contacts 436 facingaway from the chip. The first and second contacts of interposer 434 areconnected to one another by internal wiring layers 437 disposed withinthe interposer. The assembly further includes a printed wiring board(“PWB”) or third element 448. The first contacts 432 of the interposerare connected to the contacts of the chip by composite conductiveelements 422 as discussed above, each including a core and a coating oflow-melting material. The second contacts 436 of the interposer areconnected to contacts 450 on PWB 448 by conventional solder balls 451.The filler 446 surrounding the composite conductive elements in thisembodiment is not compliant, but instead is a rigid material such as ahard epoxy compound. The CTE of interposer 434 is between the CTE ofchip 424 and the CTE of PWB 448. In this embodiment, the compositeconductive elements provide fatigue-free connections between the chipand the interposer. Some physical strain may occur in filler layer 446during thermal cycling, but this will not cause fatigue of theelectrical connections. In a further variant, solder balls 451 can bereplaced by additional composite conductive elements similar tocomposite elements 422.

[0041] In still further variants, the filler surrounding the compositeelements may be omitted entirely. The cores of the composite elementswill retain the conductive material by surface tension, and will supportthe first and second elements of the assembly, when the conductivematerial is in the liquid state. Desirably, such an assembly is providedwith an external package to prevent application of substantial forces tothe first and second elements, and to protect the conductive materialfrom oxidation.

[0042] As these and other variations and combinations of the featuresdiscussed above can be employed, the foregoing description should betaken by way of illustration rather than by way or limitation of theinvention.

What is claimed is:
 1. A microelectronic assembly comprising: (a) firstand second microelectronic elements; and (b) a plurality of compositeconductive elements, each said composite conductive element including acore and a layer of a conductive material surrounding the core, saidconductive material having a melting temperature less than about 150°C., said cores having a melting temperature higher than said conductivematerial, said conductive elements being disposed between saidmicroelectronic elements and connecting said microelectronic elements toone another.
 2. An assembly as claimed in claim 1 wherein said meltingtemperature conductive material of said conductive material is less thanabout 85° C.
 3. An assembly as claimed in claim 1 wherein saidmicroelectronic elements have a normal operating temperature range andwherein said melting temperature of said conductive material is withinor below said normal operating temperature range.
 4. An assembly asclaimed in claim 1 wherein said layer of conductive material in eachsaid conductive element is about 50 μm or less.
 5. An assembly asclaimed in claim 1 wherein each said core is spherical.
 6. An assemblyas claimed in claim 1 wherein each said core is hollow.
 7. An assemblyas claimed in claim 1 wherein each said core is solid throughout.
 8. Anassembly as claimed in claim 1 wherein said cores are substantiallyrigid.
 9. An assembly as claimed in claim 1 wherein said cores arecompliant.
 10. An assembly as claimed in claim 1 wherein said secondmicroelectronic element includes a flexible film having a first surfacefacing in a first vertical direction toward said first microelectronicelement and having a second surface facing in a second verticaldirection away from said first microelectronic element.
 11. An assemblyas claimed in claim 10 wherein said second microelectronic element hascontacts on said second surface offset from said conductive elements inone or more lateral directions transverse to said vertical directions.12. An assembly as claimed in claim 11 wherein said secondmicroelectronic element has contacts on said first surface electricallyconnected to said contacts on said second surface and wherein said firstmicroelectronic element includes one or more semiconductor chips havingcontacts on a front face, said conductive elements electricallyconnecting said contacts on said front face of said one or moresemiconductor chips to said contacts on said first face of said secondmicroelectronic element.
 13. An assembly as claimed in claim 12 furthercomprising a compliant dielectric material disposed between said firstand second elements and surrounding said conductive elements.
 14. Anassembly as claimed in claim 10 wherein said second microelectronicelement has contacts on said first and second surfaces electricallyconnected to one another, and wherein said first microelectronic elementincludes one or more semiconductor chips having contacts on a frontface, said conductive elements electrically connecting said contacts onsaid front face of said one or more semiconductor chips to said contactson said first face of said second microelectronic element.
 15. Anassembly as claimed in claim 13 further comprising a compliantdielectric material disposed between said first and second elements andsurrounding said conductive elements.
 16. An assembly comprising: (a) afirst microelectronic element having contacts thereon; (b) a secondmicroelectronic element including a flexible sheet having lateraldirections along the sheet and having vertical directions transverse tothe sheet, said sheet having first contacts facing toward said firstmicroelectronic element and having second contacts facing away from saidmicroelectronic element, said second contacts being offset from saidfirst contacts in one or more of said lateral directions, at least someof said second contacts being electrically connected to at least some ofsaid first contacts; and (c) first electrically conductive elementsconnecting said first contacts to said first microelectronic element,said sheet being vertically spaced from said first microelectronicelement so that said sheet is free to flex in said vertical directions,whereby flexural movement of said sheet can accommodate movement of saidsecond contacts relative to said first microelectronic element.
 17. Anassembly as claimed in claim 16 wherein said first electricallyconductive elements support said sheet away from said firstmicroelectronic element.
 18. An assembly as claimed in claim 17 furthercomprising a compliant filler disposed between said sheet and said firstmicroelectronic element.
 19. An assembly as claimed in claim 17 furthercomprising a third microelectronic element, said sheet being disposedbetween said first and third microelectronic elements, the assemblyfurther comprising second conductive elements electrically connectingsaid second contacts and said third microelectronic element.
 20. Aconductive element including a spherical core and a layer of aconductive material on said core, said conductive material having amelting temperature less than about 150° C., said core having a meltingtemperature above the melting temperature of said conductive material.21. A conductive element as claimed in claim 20 wherein said layer ofconductive material is less than about 50 microns thick.
 22. Aconductive element as claimed in claim 20 wherein said core is hollow.23. A conductive element as claimed in claim 20 wherein said core issolid throughout.
 24. A conductive element as claimed in claim 20wherein said core and said conductive material are metallic.